FIG. 1 is a simplified block diagram 100 that illustrates a conventional technique used for transmitting a radio signal. The signal 102 at baseband (BB) or intermediate frequency (IF) is up-converted to radio frequency (RF), by mixing through the mixer 104 with a signal from the local oscillator (LO) 106. The resulting RF signal is then power amplified by the RF power amplifier 108 for transmission 110. Efficient power amplification is preferred especially for portable radio devices. Analog power amplification is inefficient in power consumption or troubled by nonlinear distortion problems. There has been a lot of effort expended to develop an efficient digital RF power amplifier that can overcome these problems. However, implementing such a digital amplifier at RF is technically very challenging because of the high sampling rates involved. Therefore, an overriding goal of radio transmitter designers is to improve the up-conversion and power amplification of radio signals.